GM Service Manual Online
For 1990-2009 cars only

Description

A three-way catalytic (TWC) converter controls exhaust emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that is present in the exhaust gas. This process will convert the HC and the CO into water vapor and carbon dioxide (CO2), and will reduce the NOx, converting the NOx into nitrogen. The catalytic converter also stores oxygen. The engine control module (ECM) monitors this process by using a heated oxygen sensor (HO2S) which is in the exhaust stream past the three-way catalytic converter. The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust emissions effectively. The ECM monitors the catalyst efficiency by first allowing the catalyst to heat, then waiting for a stabilization period while the engine is idling. Then, the ECM adds and removes fuel while monitoring the reaction of the HO2S. When the catalyst is functioning properly, the HO2S 2 response to the extra fuel is slow compared to the HO2S 1. When the HO2S 2 response is close to that of the HO2S 1, the oxygen storage capacity or efficiency of the catalyst may be degraded below an acceptable threshold. If the ECM detects the degraded condition the DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTCs:

    • DTC P0420 Catalyst System Low Efficiency Bank 1
    • DTC P0430 Catalyst System Low Efficiency Bank 2

Conditions for Running the DTC

    • DTCs P0030, P0031, P0032, P0036, P0037, P0038, P0050, P0051, P0052, P0056, P0057, P0058, P0101, P0102, P0103, P0106, P0107, P0108, P0111, P0112, P0113, P0116, P:0117, P0118, P0119, P0121, P0122, P0123, P0125, P0128, P0131, P0132, P0133, P0134, P0137, P0138, P0139, P0140, P0151, P0152, P0153, P0154, P0157, P0158, P0159, P0160, P0171, P0172, P0174, P0175, P0261, P0262, P0263, P0264, P0265, P0266, P0267, P0268, P0270, P0271, P0273, P0274, P0276, P0277, P0300, P0341, P0342, P0400, P0401, P0402, P0403, P0404, P0405, P0406, P0441, P0442, P0444, P0445, P0455, P0456, P0501, P1171, P1172, P1174, P1175, P1504 are not set.
    • Before the ECM performs the test, the vehicle must be driven under the following conditions:
    • The engine coolant temperature is more than 75°C (167°F).
    • The ignition 1 voltage is between 11-16 volts.
    • The fuel system is in closed loop.
    • The modeled catalyst temperature is between 500-900°C (932-1,652°F).
    • The vehicle speed is more than 5 Km/h (3 mph)
    • The mass air flow (MAF) is between 100-350 mg/stroke.
    • The engine speed is less than 3400 rpm.
    • The evaporative emission (EVAP) canister purge valve is not in close state.
    • No
    • The DTCs run continuously once the above conditions are met.

The catalyst test is disable if the ECM detects a sudden change in engine speed, load or throttle opening.

Conditions for Setting the DTC

The ECM has determined the catalyst efficiency has degraded below a calibrated threshold.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Test Description

The number below refers to the step number on the diagnostic table.

  1. The MIL flashes if the ECM detects a catalyst damaging misfire. If the engine is misfiring, repair the misfire condition so catalyst damage does not occur.

Step

Action

Yes

No

1

Did you perform the Diagnostic System Check - Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Observe the DTC information on the scan tool.

Are there any other DTCs set?

Go to Diagnostic Trouble Code (DTC) List

Go to Step 3

3

Inspect for the following conditions:

    • The catalytic converter is an original part.
    • Inspect the exhaust system for leaks, damage, or missing hardware in the area from the converter to the heated oxygen sensor (HO2S).
    • HO2S is secure and the wiring is not damaged or contacting the exhaust.

Did you find and correct the condition?

Go to Step 5

Go to Step 4

4

Important: Before replacing the 3-way catalytic converter (TWC), correct any conditions that may have damaged the converter.

Replace the three-way catalytic converter. Refer to Catalytic Converter Replacement in Engine Exhaust.

Did you complete the replacement?

Go to Step 5

--

5

Was the customer's concern that the malfunction indicator lamp (MIL) flashes?

Go to DTC P0300

Go to Step 6

6

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

Go to Step 2

Go to Step 7

7

Observe the Capture Info with a scan tool.

Are there any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK